Apparently, Matt Dillion has decided to roll his own filesystem for DragonFly. "Here is my initial outline of the filesystem design. It is open for discussion. Please feel to ask questions for anything you do not understand. I do not intend to start coding anything for at least two weeks. There are currently two rough spots in the design. First, how to handle segment overflows in a multi-master environment. Such overflows can occur when the individual masters or slaves have different historical data retention policies. Second, where to store the regeneratable indexes."

DragonFly has a very specific set of goals, which include high scalability and transparent clustering. From the feature set of the filesystem, it's clear that its desigened specifically to meet these goals. Read the specs for yourself, it's a very different best. It doesn't look like any filesystem I've encountered, and certainly not like {JFS, XFS, ZFS, BFS, HFS+, NTFS, EXT3/4, REISER4, UFS2}, which would be the main alternative mature and proven choices.